1. Field of the Invention
The present invention relates generally to a process for manufacturing preforms used in optical fibers, and more specifically to manufacturing preforms by outside reactive deposition.
2. Information Disclosure Statement
The prior art regarding reactive deposition essentially centers around two methods: inside reactive deposition and outside reactive deposition. Several patents disclose methods for the inside coating of a core material For example, Greenham et al., U.S. Pat. No. 4,936,889, Moisan et al., U.S. Pat. No. 4,944,244, Auwerda U.S. Pat. No. 4,714,589, Nourshargh U.S. Pat. No. 4,619,680, and European Patent DE-PS 24 44 100 are exclusively concerned with inside deposition methods. Additionally, European Patent DE-OS36 32 684 discloses a resonator design useful for performing an inside reactive deposition.
The inside reactive deposition process involves generating a plasma zone inside a tube, and axially moving the tube relative to the plasma generating equipment. The reactive deposition is performed in a pressure range of 1 to 10 Torr, and a temperature zone is superimposed over the plasma zone. Because the reaction chamber is small, this process provides good control of the reactive gas and maintains a clean depositing environment.
The product obtained from the process is a preform which is used to draw an optical fiber. For data communication applications, the light conducting core of the fiber is usually small in comparison with the outside diameter of the fiber. The process of inside deposition (as described by the above mentioned patents) is suitable for this application.
Other applications such as laser delivery or sensing systems, however, require fibers with a large, usually undoped core and a relatively thin, mostly fluorine doped cladding. For these applications, it is preferred to start the manufacturing process from an undoped, commercially available quartz glass rod of high purity and to deposit only a relatively thin doped layer on its outside. The inside deposition method on the other hand requires a high quantity of material to be deposited in this case because the core area is large. Consequently, this method consumes a large part of the gas mixture.
The manufacturing of preforms with high core to clad ratios is usually performed starting from undoped quartz rods. These rods are then coated on the outside with a doped quartz layer by means of atmospheric plasma burners. This process suffers from several shortcomings such as environmental contamination and the elaborate measures required to maintain purity of the deposited substance in the open atmosphere. Consequently, the atmospheric plasma burner yields much lower reagent disassociation and deposition efficiencies of silica. For example, Mansfield, U.S. Pat. No. 4,863,501 describes an atmospheric plasma burner depositing soot, not fused silica.
One method that avoids a plasma burner is described in AJlG (DE 3331899 A1). That patent describes a method for the reactive deposition on a glass-body. More specifically, it describes a method for the production of preforms where the essential advantage claimed is the absence of moving parts. This is achieved by inserting a rod in the center of a discharge tube and filing the tubes with gas. Next, a theta pinch plasma reaction is initialized to compress the ionized gas particles towards the rods surface where layers are successively deposited.
This process, however, has several shortcomings. A major disadvantage of this method is a build-up on the inside of the tube and not the rod. This results because the inductive discharge produces maximum field strength on the inside of the tube. Additionally, the extremely strong ionization that would be required to produce the radial compression toward the rod would inhibit the chemical process necessary for effective deposition. Finally, the high field strength required would make a continuous process impractical and perhaps impossible.
Therefore, a need exists to deposit layers on the outside of a glass-body, to maintain high gas efficiencies, to avoid contaminating layers, to minimize build up on the inside of the device, and to provide for possible continuous processing. The present invention fulfills these needs. SUMMARY OF THE INVENTION
The present invention is directed at an improved method for reactive deposition in the production of preforms. It is a particular object of the invention to deposit layers on the outside of a starting body while maintaining high gas efficiencies, avoiding contamination of the layers, and allowing for a continuous process. This is accomplished by using a wave type resonator in a controlled atmosphere. The resonator in the present invention avoids the extra consolidation stage necessary in the Mansfield patent because the system works at low pressures (several Torr) and the resonator produces a much higher field intensity (E) than a burner as described in the Mansfield patent. Since deposition efficiency strongly increases with field intensity (E), the present invention promotes higher deposition rates of fused silica glass.
It is another object of the invention to deposit on the outside of a starting body while suppressing deposition on the inside walls of the tube. The invention achieves this object by achieving a maximum field strength in the region of the starting body and a node in the field strength near the tube. Since the deposition relates to field strength, the layers would tend to form on the starting body rather than the inside of the device. Alternatively or in combination, the present invention maintains a radial protective gas flow through (a porous) tube which buffers the inner wall from the reactive gas.